Update gtest.rb

I've added a compilation flag "CPPFLAGS=-DGTEST_HAS_TR1_TUPLE=0" to be able to build gtest on OSX 10.9 / Xcode 5.0.1.  I am not sure how to obtain MacOS version, then I would have added 'if' condition. Please test it on previous versions of OSX.
Merge pull request #5 from Lenskiy/patch-1

Update gtest.rb